What determines the size of sand or cobbles on a beach?
Two things determine the size of sand on beaches: the energy (height) of the waves, and the size of the material furnished to the beach. Other things being equal, the higher the average waves on a beach, the coarser the sand. This is because higher waves tend to move the smaller grains offshore to quieter water. Dune sand is consistently finer grained than the adjacent beaches even though the dune sand is derived from winds blowing across the beach. The difference in size is due to the fact that winds generally only pick up and carry the finer grains of beach sand and leave the heavier, larger sand grains behind. Grain size determines the slope of the beach. Beaches with finer grain sizes tend to be flatter with gentler slopes (between the high and low tide line) than beaches with coarser grains. The reason for this has to do with the ability of sand to absorb water (porosity and permeability) from wave swash. Fine sand absorbs relatively little water and most of the water that flows u
